The use of Ziegler-Natta catalyst systems to promote various olefin polymerizations is well known. These catalyst systems typically include a catalyst precursor comprising one or more of each of a transition metal, an electron donor or ligand and a cocatalyst. Complete or near complete activation of the precursor is necessary in order to obtain the high-level of catalyst activity required for a commercial olefin polymerization process. Catalyst activation can consist of multiple steps which may include chlorination, reduction reactions, displacement of internal electron donors or ligands and other chemical modifications to the catalyst precursor that are necessary to obtain high levels of catalyst productivity. Other catalyst attributes are also affected by the catalyst activation process; such as stereoregulation (for propylene and butene polymerization), molecular weight distribution, comonomer incorporation and the like. It is well known in the art that these key catalyst attributes are affected by a number of variables, including the method of catalyst manufacture or formation, the use of internal electron donors, the chemical composition of the internal electron donor, the use of external electron donors and the amount of the electron donors present.
Activation of the catalyst precursor requires the removal of the internal electron donor from the vicinity of the active site, i.e., the metal, and, if necessary, reduction of the metal. The activator extracts the internal electron donor compound from the active site in one of several ways. The internal donor can be removed by complex formation, typically with a Lewis Acid, or by alkylation or by reduction and alkylation if the valence state of the metal requires reduction. Typical activating compounds are Lewis Acids.
Activation of the catalyst precursor may occur by (i) full activation in the polymerization reactor by the cocatalyst, (ii) partial activation before introduction of the precursor into the reactor and completion of the activation in the reactor by means of the cocatalyst, or (iii) full activation prior to introduction of the precursor into the reactor. There are several advantages and disadvantages to all three techniques.
Complete activation of the catalyst inside the polymerization reactor typically requires a substantial excess of activator compound and in the case of higher (C3, C4 and up) olefin polymerizations, use of excess selectivity control agent. Advantages to this technique are its simplicity of catalyst manufacture and feed. However, excess activator compound is not only an added operational expense, but it may cause operational problems or detriment to the final product. In addition, there is no way to modify the catalyst composition in an on-line fashion to significantly affect the polymerization response.
Partial activation of the catalyst precursor outside of the reactor requires additional process steps and equipment followed by final activation in the reactor (which, again, requires the use of excessive amounts of activator). Partial activation outside of the reactor also results in the need to store the partially activated catalyst and the likelihood of catalyst deactivation during storage, either due to continued reactions with the activator compounds and their reaction products or due to impurities invariably present in inert gases (such as nitrogen) typically used to blanket these catalysts during storage. Although the formulation of the catalyst may be changed in the partial activation procedure external to the reactor, this again results in a static catalyst formulation. Any polymerization response changes can only come from changing the catalyst batch. Currently, there is no on-line control technique available for fine control of the molecular weight distribution of desired polymers.